Mutations of the adenomatous polyposis coli (APC) gene have recently been shown to play an important role in colorectal tumorigenesis. We investigated mutations of the APC gene in 30 gastric adenomas obtained endoscopically. Mutations of the APC gene were examined by polymerase chain reaction-single-strand conformation polymorphism analysis followed by sequencing of the polymerase chain reaction products. Mutations were detected in 20% (6 of 30) of gastric adenomas. In addition, deletion of the remaining allele that subsequently led to complete inactivation of the APC gene was confirmed in one-half (3 of 6) of the tumors with APC gene mutations. Sequencing analysis confirmed that the mutations resulted in truncation of the gene products or in an amino acid change. The incidences of mutations of the APC gene remained constant regardless of the size or degree of histological atypia. Our observations suggest that mutations of the APC gene, similarly to those in colorectal tumorigenesis, occur during the early stages of gastric adenoma development.